routing: consolidate final hop params for newRoute

This commit creates a wrapper struct, grouping all parameters that
influence the final hop during route construction. This is a preliminary
step for passing in the receiver's invoice feature bits, which will be
used to select an appropriate payment or payload type.

routing/payment_session.go

@@ -129,8 +129,12 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	// a route by applying the time-lock and fee requirements.
 	sourceVertex := route.Vertex(ss.SelfNode.PubKeyBytes)
 	route, err := newRoute(
-		payment.Amount, sourceVertex, path, height, finalCltvDelta,
-		payment.DestCustomRecords,
+		sourceVertex, path, height,
+		finalHopParams{
+			amt:       payment.Amount,
+			cltvDelta: finalCltvDelta,
+			records:   payment.DestCustomRecords,
+		},
 	)
 	if err != nil {
 		// TODO(roasbeef): return which edge/vertex didn't work